B. Sundelius

OJ 287 - binary pair of supermassive black holes

A historical light curve of the BL Lacertae object OJ 287 is constructed in the optical V band using observations between 1890 and the present. The curve exhibits periodic outbursts at intervals of 11.6 yr or 9 yr. The individual outbursts show an initial maximum with decaying submaxima at intervals of about 1 month. It is proposed that these characteristics are caused by a binary in the nucleus of OJ 287, with the binary members having an accretion disk around them. The outbursts at 9-yr intervals would be caused by the tidal action of the companion on the disk of the larger black hole. A computer simulation is used to demonstrate that the inflow into the center of such a disk during the repeated periastron passages of a companion will produce an outburst similar to the ones observed. Based on indirect evidence, probable masses of 5 billion solar and 20 million solar are determined for the two objects. 38 references.

A computer-generated galaxy model with long-lived two-armed spiral structure

A long-lived two-armed spiral has been generated in an N-body computer simulation of a galaxy with a static bulge and halo and an active disk composed of 60,000 particles. The spiral lasts for about three pattern revolutions without severe distortion and persists for at least two more revolutions with distortions and bifurcations resulting from an increasingly clumpy ISM. This suggests that two-armed grand design spirals in nonbarred noninteracting galaxies can be long-lived if star formation and other heat sources not present in the simulation maintain a steady interstellar medium.

Formation of leading spiral arms in retrograde galaxy encounters

We study, theoretically and withN-body simulations, the formation of spiral structures in retrograde galaxy encounters. A one-armed leading spiral dominates in a disc if (i) the tidal perturbation from the retrograde companion is large enough, and (ii) the disc is surrounded by a massive halo. From the literature we find that very few spirals in a sample of galaxies with a large companion have leading spiral arms. A possible reason for this is that very few spiral galaxies have a halo with a larger mass than the disc mass.